Influenza is once again at the center of health concerns in Europe. In just one week, cases have risen by 103%, driven mainly by an increase in infections among children and adolescents. The main cause of this spike is the circulation of influenza A virus subclade K (H3N2), a variant characterized by a higher transmission capacity and the potential to partially circumvent previous immunity, but without causing more severe cases.
The most recent data from the Instituto de Salud Carlos III (ISCIII) confirm that the 2025-26 flu season is getting earlier and more intense than usual. Between December 1 and 7, the rate of acute respiratory infections detected in Primary Care went from 641.8 to 840.1 cases per 100,000 inhabitants, with influenza clearly in the lead against covid-19 and Respiratory Syncytial Virus (RSV).
Unusual increase in cases in the run-up to winter
The increase in influenza is especially significant in the field of Primary Care. According to the ISCIII, the number of cases detected rose in one week from 172 to 349.5 infections per 100,000 inhabitants, an increase of 103.2%. This growth is concentrated, above all, in children and adolescents, a pattern that reinforces the idea that the flu season is earlier than in previous years.
General practitioners had already warned weeks ago of this upturn. Influenza is traditionally the respiratory virus that puts most pressure on healthcare systems, due to the increase in hospitalizations and the impact on mortality among vulnerable people. If the current transmission rate associated with the K variant is maintained, this season could be among the most intense in recent years.
What is Influenza K and why is it more easily spread?
Subclade K belongs to the influenza A virus (H3N2) and is defined as a “viral recombination” with relevant differences with respect to previous strains. The Spanish Society of Family and Community Medicine (semFYC) warned weeks ago of its early circulation in December, something unusual for this time of year.
José María Molero, spokesman for the Working Group on Infectious Diseases of the semFYC, explained that this variant has been circulating since spring in different European countries, which made its arrival in Spain inevitable, since “viruses cannot be confined within a country”. In Europe and Asia, the K subclade already represents about half of the sequences analyzed in some territories.
In Japan, where its prevalence was confirmed, the epidemic peak was earlier and a higher volume of hospital admissions was recorded. Even so, specialists stress that this is not a more serious variant, but a more transmissible one.
Atypical behavior of respiratory viruses
The current season is marked by an unusual coincidence: the simultaneous advance of influenza, covid-19 and RSV. According to family physicians, “the current respiratory virus season is showing atypical behavior: European surveillance data and the clinical evolution observed in consultations indicate an advance of approximately one month in the onset of influenza activity, together with a simultaneous increase in infections by RSV and other respiratory viruses”.
This scenario has led experts to insist on the need to reinforce virological surveillance and prepare health services for an expected increase in cases during the month of December.
Why the K-variant can infect even vaccinated people
If the K subclade ends up dominating the season, the situation becomes more complicated from the epidemiological point of view. The virus presents alterations in the hemagglutinin protein, key for binding to human cells, which allows it to “escape” more easily from the immune system’s defenses.
This explains why it can cause symptoms even in people who have been vaccinated or who have previously had the flu. However, specialists insist that this greater contagiousness does not imply greater clinical severity.
Masks, tests and assistance pressure
The rapid spread of influenza has led several autonomous communities to recommend or impose the use of face masks in healthcare centers and closed spaces. Murcia and Catalonia have already adopted this measure as mandatory in certain areas.
According to the Cofares Trend Observatory, demand for face masks has increased by 385%, while antigen tests have grown by 150%. Products to relieve cold and flu symptoms have also seen a 47% increase.
Vaccination remains key, experts say
Despite the circulation of subclade K, the semFYC reminds that vaccination remains essential. Although protection against this variant may be lower, the vaccine continues to be effective in preventing serious infections caused by other viruses included in its composition.
The World Health Organization (WHO) has endorsed this message in a recent statement: “Although there are some genetic differences between circulating influenza viruses and the strains included in the vaccines, the seasonal influenza vaccine can still provide protection against derived viruses and other viral strains included in the vaccine”.
WHO further stresses that vaccination “protects against severe disease and remains one of the most effective public health measures,” while acknowledging that its efficacy against clinical disease associated with subclade K remains “uncertain.”
Recommendations in the face of the spread of influenza
Specialists insist on keeping calm and applying basic prevention measures: strengthening the immune system, avoiding unnecessary contacts if there are symptoms, extreme hand hygiene and staying at home when sick. Most people recover in one or two weeks without the need to visit the doctor.
Annual vaccination continues to be the main prevention tool, especially in people over 60 years of age, young children, pregnant women, chronic patients, healthcare personnel and vulnerable groups. The vaccine is safe, does not spread the flu and its adverse effects are usually mild and transitory.
